Re: [PATCH] MIPS: pci: lantiq: restore reset gpio polarity

[Martin Schiller <ms@dev.tdt.de>]

On 2024-06-13 22:06, Hauke Mehrtens wrote:
> On 6/12/24 21:47, Martin Schiller wrote:
>> On 2024-06-12 20:39, Martin Schiller wrote:
>>> On 2024-06-12 19:47, Dmitry Torokhov wrote:
>>>> Hi Marton,
>>> 
>>> Hi Dmitry,
>>> 
>>>> 
>>>> On Fri, Jun 07, 2024 at 11:04:00AM +0200, Martin Schiller wrote:
>>>>> Commit 90c2d2eb7ab5 ("MIPS: pci: lantiq: switch to using gpiod 
>>>>> API") not
>>>>> only switched to the gpiod API, but also inverted / changed the 
>>>>> polarity
>>>>> of the GPIO.
>>>>> 
>>>>> According to the PCI specification, the RST# pin is an active-low
>>>>> signal. However, most of the device trees that have been widely 
>>>>> used for
>>>>> a long time (mainly in the openWrt project) define this GPIO as
>>>>> active-high and the old driver code inverted the signal internally.
>>>>> 
>>>>> Apparently there are actually boards where the reset gpio must be
>>>>> operated inverted. For this reason, we cannot use the 
>>>>> GPIOD_OUT_LOW/HIGH
>>>>> flag for initialization. Instead, we must explicitly set the gpio 
>>>>> to
>>>>> value 1 in order to take into account any "GPIO_ACTIVE_LOW" flag 
>>>>> that
>>>>> may have been set.
>>>> 
>>>> Do you have example of such boards? They could not have worked 
>>>> before
>>>> 90c2d2eb7ab5 because it was actively setting the reset line to 
>>>> physical
>>>> high, which should leave the device in reset state if there is an
>>>> inverter between the AP and the device.
>>> 
>>> Oh, you're right. I totally missed that '__gpio_set_value' was used 
>>> in
>>> the original code and that raw accesses took place without paying
>>> attention to the GPIO_ACTIVE_* flags.
>>> 
>>> You can find the device trees I am talking about in [1].
>>> 
>>> @Thomas Bogendoerfer:
>>> Would it be possible to stop the merging of this patch?
>>> I think We have to do do some further/other changes.
>>> 
>>>> 
>>>>> 
>>>>> In order to remain compatible with all these existing device trees, 
>>>>> we
>>>>> should therefore keep the logic as it was before the commit.
>>>> 
>>>> With gpiod API operating with logical states there's still 
>>>> difference in
>>>> logic:
>>>> 
>>>>     gpiod_set_value_cansleep(reset_gpio, 1);
>>>> 
>>>> will leave GPIO at 1 if it is described as GPIO_ACTIVE_HIGH (which 
>>>> is
>>>> apparently what you want for boards with broken DTS) but for boards
>>>> that accurately describe GPIO as GPIO_ACTIVE_LOW it well drive GPIO 
>>>> to
>>>> 0, leaving the card in reset state.
>>>> 
>>>> You should either use gpiod_set_raw_value_calsleep() or we can try 
>>>> and
>>>> quirk it in gpiolib (like we do for many other cases of incorrect 
>>>> GPIO
>>>> polarity descriptions and which is my preference).
>> 
>> So you mean we should add an entry for "lantiq,pci-xway" to the
>> of_gpio_try_fixup_polarity()?
>> Do you know any dts / device outside the openWrt universe which is 
>> using
>> this driver.
>> 
>> For the lantiq targets in openWrt, the devicetree blob is appended to
>> the kernel image and therefore also updated when doing a firmware
>> upgrade. So, maybe it would also be an option to fix the driver (using
>> GPIO_ACTIVE_* flag for the initial level and set it to 0 -> 1 -> 0) 
>> and
>> rework all the dts files to use GPIO_ACTIVE_LOW.
>> 
>> Then we won't need any quirks.
> 
> I am not aware that anyone is using a recent kernel on the VRX200
> outside of OpenWrt. I am also not aware that anyone is *not* appending
> the DTB to the kernel. The SoC is pretty old now, the successor of
> this SoC was released about 10 years ago.
> 

We're not just talking about VRX200 (VR9) here, but even older devices
such as AR9 and Danube.

> For me it would be fine if you fix the broken device device trees
> shipped with the upstream kernel and with OpenWrt to make them work
> with the PCI driver instead of investing too much time into handling
> old DTBs.
> 
> The PCI reset is inverted on some boards to handle a dying gasp. If
> the power breaks down the reset should get triggered and the PCIe
> device can send a dying gasp signal to the other side. This is done on
> the reference designs of some Lantiq PCIe DSL card for the VRX318 and
> probably also some other components.
> 
> Hauke

What I missed so far is the fact that the driver used '__gpio_set_value'
before Dmitry's commit and thus used raw access to the GPIO.

This effectively means that every device that has worked with the driver
so far must have an ACTIVE_LOW reset, no matter what was configured in
the device tree.


So renaming the property in the dts from "gpio-reset" to "reset-gpios"
and setting the FLAGS to "GPIO_ACTIVE_LOW" should actually solve the
problem.

What still bothers me about the driver itself are 2 things:
1. the initial value of GPIOD_OUT_LOW. This means that there is no real
    defined HIGH -> LOW -> HIGH on reset.
2. if we change 1., then I think "mdelay(1)" is too short and we would
    have to change it to at least "mdelay(100)".

Martin